Memory Limits This planar has a 16MB limit because of the 386SX processor.

8557SX Bus Adapter

If you have a fantasy about swapping a Lacuna riser and
planar into your 57SX, forget it. The Lacuna riser has two mounting slots
where the screws are both horizontal, while the 57 has a vertical screw
and a horizontal screw. The lacuna mounting holes at the front don't match.

Dotted area on J1- solder pads for
a BVE segment

J10- Solder pads for a 16 pin header

J2 AVE slot

VRAM Functionality?> This one has actual sockets installed though, not empty pads...?

Yeah - most have them ... and they are somewhat connected
to the video-chip glue logic on the planar. But basically they are non-functional.
If you look on the board you will find the same old boring Inmos RamDAC
as on the Mod. 50/60 55/65 70/80, with limited bandwidth. Just enough for
standard-VGA or 320x200 / 256 colors CGA mode. And - as said: the machine
lacks the video modes in its BIOS AFAIK.

Some of the later Greenock-variants of the 8556/8557 planar even lack
these sockets and have only soldering spots. That was what my comment refered
on.

You might however try to install some chips there for 512K and see if
you can enable the mode 2Eh - that's 640 x 480 / 256 colors from a 256K
palette at 60Hz vertical / 31.5 KHz horizontal. The system should choke
out at Mode 12h ( 640 x 480 / 16 out of 256K, 60Hz / 31.5 KHz) and 13h
respectively (320 x 200 / 256 out of 256K, 70Hz / 31.5KHz).

Very friendly greetings from Peter in Germany
http://members.aol.com/mcapage0/mcaindex.htm